1. Field of the Invention
This invention relates to a liquid sealed type bushing used for an automobile engine mount and the like.
2. Description of the Prior Art
Liquid sealed type bushings are known, which comprise a generally cylindrical shaped outer cylinder and a core member arranged therein, an elastic member interposed between the outer cylinder and the core member, wherein a plurality of recesses are provided as liquid chambers on an outer peripheral side of a middle portion of the elastic member. A partition member divides two adjoining liquid chambers and is formed by a part of the elastic member and a medium to high frequency device, which includes an umbrella shaped member projecting into a liquid chamber and supported by the core member side.
According to this construction, because the partition member is rather thick (almost as thick as a minimum thickness of a circular wall portion formed on both ends of the core and the outer cylinder in an axial direction) and the outer peripheral portions thereof are directly or indirectly secured by baking to the inside of the outer cylinder, as shown in a characteristic curve 1 of FIG. 6, the dynamic spring characteristic curve possesses a minimum value B and a large peak C generated by a reaction thereof.
A reason why the minimum value B and the peak C are generated is described referring to FIG. 7-A to FIG. 7-C. FIG. 7-A is a view showing a change of a dynamic spring constant by a membrane resonance vs. frequency. The membrane resonance is generated in that the partition member conducts as an elastic membrane against an inside liquid flow, because the partition member is thinner than the peripheral circular wall portion or other portion. The axis of abscissa shows frequency and the axis of ordinate shows a dynamic spring constant.
In these figures, a characteristic curve 4 shows a conventional plot. Here, since the partition member is rather thick and is secured on the outer cylinder side, the characteristic shows the relatively high dynamic spring constant, and possesses a peak p1 and a minimum value b1 changing abruptly around a border area of the medium frequency region and the high frequency region by membrane resonance.
FIG. 7-B shows a change of a liquid column resonance vs. frequency by the medium to high frequency device. The axis of abscissa shows frequency and the axis of the ordinate shows a dynamic spring constant. In the drawing, a characteristic curve 6 shows a conventional plot, the characteristic possesses a minimum value b2 owing to a liquid column resonance on a medium frequency region close to the high frequency region, and the frequency of the minimum value b2 is arranged to be almost the same as that of the peak p1.
When the characteristic curves 4 and 6 are overlapped, characteristic curve 1 of FIG. 7-C results, namely, the peak p1 generated by the membrane resonance is cancelled by the minimum value b2 of the liquid column resonance, so the minimum value B is yielded.
Even if the minimum value of the dynamic spring constant is generated as above by using the medium to high frequency device, as shown in FIG. 7B, the peak p2, which results from the reaction of the resonance by the medium to high frequency device, remains as the peak C in the characteristic curve 1. Because of the extremely high dynamic spring constant in the higher frequency region, it cannot be realized to make the low dynamic spring characteristics in the wide frequency range across the medium to high frequency range.
On the other hand, in recent years, it is required for such a liquid sealed type bushing to actualize the low dynamic spring constant in the wide range of the medium frequency region (40 to 500 Hz), especially more than the 100 Hz region, and the high frequency region (500 to 1000 Hz).